Cellular radiotelephone systems are increasingly being used for wireless mobile communications. As known to those having skill in the art, a cellular radiotelephone system is a wide area communications network which utilizes a frequency reuse pattern in a plurality of cells. The design and operation of an analog cellular phone system are described in an article entitled "Advanced Mobile Phone Service" by Blecher, IEEE Transactions on Vehicular Technology, Vol. VT 29, No. 2, May 1980, pp. 238-244. The analog mobile cellular system is also referred to as the "AMPS" system.
Recently, digital cellular radiotelephone systems have also been proposed and implemented using a Time-Division Multiple Access (TDMA) architecture. Standards have also been set by the Electronics Industries Association (EIA) and the Telecommunications Industries Association (TIA) for an American Digital Cellular (ADC) architecture which is a dual mode analog and digital system following EIA/TIA standard IS-54B. Telephones which implement the IS-54B dual mode architecture are presently being marketed by the assignee of the present invention. Different standards have been promulgated for digital cellular radiotelephone systems in Europe. The European digital cellular system, referred to as GSM, also uses a TDMA architecture.
A cellular radiotelephone network includes a plurality of spaced-apart small radio zones called cells. Each cell includes a base station for transmitting and receiving messages to and from cellular radiotelephones which are located within the cell range. A plurality of base stations are connected to a Mobile Telephone Switching Office (MTSO) which acts as a controller for the cellular system. The MTSO is also connected to the Public Switched Telephone Network (PSTN) to allow communications between cellular radiotelephones and wire line phones.
As used herein, the term "cellular radiotelephone" encompasses a wide variety of portable radiotelephone devices which access a cellular radiotelephone system. Cellular radiotelephones include portable telephones of the hand-held or bag phone variety and permanently mounted car cellular telephones. The term "cellular radiotelephone" also includes terminals which provide functions in addition to those of a cellular telephone, such as facsimile, data communications, data processing, word processing applications and other personal communications systems functions. These high function cellular radiotelephones are often referred to as "Personal Communications Systems" (PCS).
It is often desirable for the current time of day to be available at the cellular radiotelephone. For example, it is often desirable for the cellular radiotelephone to display a current time of day, either continuously or in response to a user request. In another example, a cellular radiotelephone may store an incoming message when the cellular radiotelephone is unattended. In this situation, it is desirable to timestamp the incoming message with a current time of day. Other functions at the cellular radiotelephone may also make use of the current time of day.
In order to provide a current time of day at a cellular radiotelephone, some digital cellular radiotelephone systems broadcast a current time of day as part of their overhead messaging. However, the analog (AMPS) system and the dual mode analog and digital system do not typically broadcast a current time of day as part of overhead messaging. Accordingly, in order to implement a current time of day, cellular radiotelephones which operate under an analog or dual mode standard have included a real time clock chip. Unfortunately, real time clock chips may add to the cost of the cellular radiotelephone, may increase the power consumption thereof and may make miniaturization of the cellular radiotelephone more difficult. Accordingly, there is need for systems and methods which generate a current time of day in a cellular radiotelephone without the need for a real time clock chip.